Atomic spectroscopy using broadband ultrashort laser pulses with durations in the 100-fs range seems nearly impossible because the large bandwidth of the pulses exceeds the line-width of atomic transitions by several orders of magnitude. Therefore the energy loss of ultrashort pulses crossing a narrow-linewidth medium is very low, even if the center frequency of the pulses is tuned to the transition frequency. For instance, the total absorption of a 100-fs pulse centered at the D/sub 2/-absorption line of Rb near 780 nm is well below 10/sup -3/ if the pulse is sent through a 75-mm-long Rb vapor cell at room temperature. Much more information can be obtained by measurement of the change of the pulse phase rather than the pulse energy after the pulse has crossed the medium. We found that a measurement technique similar to previously published second harmonic generation (SHG) frequency-resolved optical gating (FROG) is very convenient for that purpose. In our first experiments we send light pulses with a duration of 100 fs, a Fourier-limited bandwidth of 10 nm, and a pulse energy of 1 nJ through a 75-mm long Rb vapor cell. The pulses were analyzed with use of a SHG-FROG system composed on a single shot correlator and dispersive optics that depicted temporal and spectral information as two-dimensional images on a CCD sensor. The SHG-FROG signals depended strongly on the Rb vapor pressure inside the cell, which was controlled by the cell temperature. While these experiments analyzed steady states, the new technique principally allows the monitoring of transient states as well. We present first results on the investigation of optical pumped material.
Atomic spectroscopy using broadband ultrashort laser pulses with durations in the 100-fs range seems nearly impossible because the large bandwidth of the pulses exceeds the line-width of atomic transitions by several orders of magnitude. Therefore the energy loss of ultrashort pulses crossing a narrow-linewidth medium is very low, even if the center frequency of the pulses is tuned to the transition frequency. For instance, the total absorption of a 100-fs pulse centered at the D/sub 2/-absorption line of Rb near 780 nm is well below 10/sup -3/ if the pulse is sent through a 75-mm-long Rb vapor cell at room temperature. Much more information can be obtained by measurement of the change of the pulse phase rather than the pulse energy after the pulse has crossed the medium. We found that a measurement technique similar to previously published second harmonic generation (SHG) frequency-resolved optical gating (FROG) is very convenient for that purpose. In our first experiments we send light pulses with a duration of 100 fs, a Fourier-limited bandwidth of 10 nm, and a pulse energy of 1 nJ through a 75-mm long Rb vapor cell. The pulses were analyzed with use of a SHG-FROG system composed on a single shot correlator and dispersive optics that depicted temporal and spectral information as two-dimensional images on a CCD sensor. The SHG-FROG signals depended strongly on the Rb vapor pressure inside the cell, which was controlled by the cell temperature. While these experiments analyzed steady states, the new technique principally allows the monitoring of transient states as well. We present first results on the investigation of optical pumped material.
A robust approach for acquiring background-free, multitransition absorption spectra under single-laser-shot conditions is demonstrated using broadband, ultrashort laser pulses. This technique—referred to as time-resolved optically gated absorption (TOGA)—exploits the inherent differences in timescales between broadband, femtosecond-duration light sources and the longer-duration responses of narrowband atomic or molecular absorption features. An optical temporal gate, based on frequency mixing via sum-frequency generation or difference-frequency generation, is used to isolate these long-lived time-domain absorption features from the ultrashort component associated with the broadband absorption light source. A proof-of-principle demonstration of TOGA is provided using atomic Rb as an absorbing medium. Application of this technique toward single-laser-shot simultaneous detection of hydroxyl radical concentration and the corresponding local temperature is also demonstrated in a reacting flow. These results indicate that TOGA can provide spectrally resolved, broadband, background-free absorption measurements at laser-source repetition rates.
The role of weakly chirped pulses (time bandwidth product, DeltanuDeltatau<0.61) on three-pulse photon echo signals has been systematically studied. Pulses with varying chirp were characterized with frequency resolved optical gating (FROG) and used to measure spectrally resolved three-pulse photon echoes of a dye in solution. The weakly chirped pulses give rise to markedly different echo signals for population times below approximately 100 fs. The chirped pulses can decrease or enhance spectral signatures of an excited state absorption transition in the echo signal. Furthermore, the observed dephasing dynamics depend on the phase of the electric fields. Simulations based on a three-level model and the electric fields retrieved from the FROG traces give a good agreement for photon echo experiments with both transform limited and chirped pulses. The simulations also allow for a numerical investigation of effects of chirp in two-dimensional spectroscopy. For a two-level system, the chirped pulses result in nonelliptical two-dimensional spectra that can erroneously be interpreted as spectral heterogeneity with frequency dependent dephasing dynamics. Furthermore, chirped pulses can give rise to "false" cross peaks when strong vibrational modes are involved in the system-bath interaction.
Offering terahertz of bandwidths and femtosecond timescales, ultrafast optics is enabling both the study of fundamental quantum optical phenomena and the advancement of quantum-enhanced applications. However, unlocking the full potential of ultrafast quantum optics requires accessing the temporal characteristics of ultrashort quantum pulses across ultrabroad bandwidths. This is particularly important in the near-infrared and visible range of the optical spectrum, which, unlike the terahertz and long-wave infrared, has remained beyond the reach of current techniques. Here, we break this barrier by translating frequency-resolved optical gating (FROG), a widely used technique for ultrafast classical pulse characterization, to the quantum regime. We show how such a quantum FROG can measure complex temporal modes and sub-optical-cycle quadrature covariances in the near-infrared, enabling complete characterization of microscopic Gaussian states. We experimentally use the quantum-FROG to report the measurement of quadrature correlations, complex temporal modes, and squeezing levels of multimode ultrafast squeezed vacuum states generated on a nanophotonic chip. We access multimode squeezing levels of a femtosecond quantum pulse approaching 7 dB and demonstrate FROG-based measurement bandwidths exceeding 100 THz. Quantum FROG enables measurement of previously inaccessible quantum features of ultrashort pulses at the sub-optical-cycle regime and highlights a practical path to accessing terahertz of bandwidths in quantum optics for applications in computing, sensing, and imaging.
Intense twin beams generated by high-gain parametric downconversion exhibit simultaneous correlations in time and frequency. To characterize the simultaneous correlations, it is important to determine the temporal correlation profile and the corresponding phase. Although sum frequency generation (SFG) is often used to characterize the temporal correlation of the twin beams, the SFG spectra reconstruct the pump intensity spectrum shape used to generate the twin beams, so useful information for phase reconstruction cannot be obtained from the SFG spectra. We propose and demonstrate frequency-resolved optical gating measurement for ultranarrow temporal correlation of twin beams (referred to as quantum FROG measurement). Quantum FROG measurement for twin beam correlations is realized using a cross-correlator based on four-wave mixing (FWM). We show theoretically that although the quantum FROG trace exhibits a different functional form obtained by the conventional FROG trace for ultrashort pulses, it is sensitive to phase dispersion. In experiments, we observed a shift of the FWM frequency, i.e., frequency chirping, within a correlation time of hundreds of femtoseconds, and the sign of the chirp was also identified.
The utilization of atomic or molecular spectroscopy for frequency locking of single-frequency laser to improve laser frequency stability plays an important role in the experimental investigation of optically pumped atomic magnetometers, atomic clocks, laser cooling and trapping of atoms, etc. We have experimentally demonstrated a technique for frequency stabilization of a single-frequency laser employing the bright state spectroscopy (BSS) with a rubidium atomic vapor cell. By utilizing the counter-propagating dual-frequency 795 nm laser beams with mutually orthogonal linear polarization and a frequency difference of 6.834 GHz, which is equal to the hyperfine splitting of rubidium-87 ground state 5S1/2, an absorption-enhanced signal with narrow linewidth at the center of Doppler-broadened transmission spectroscopy is observed when continuous scanning the laser frequency over rubidium-87 D1 transition. This is the so-called BSS. Amplitude of the absorption-enhanced signal in the BSS is much larger compared with the conventional saturation absorption spectroscopy (SAS). The relationship between linewidth and amplitude of the BSS signal and laser beam intensity has been investigated. This high-contrast absorption-enhanced BSS signal has been employed for the laser frequency stabilization. The experimental results show that the frequency stability is 4.4×10−11 with an integration time of 40 s, near one order of magnitude better than that for using the SAS.
The four-wave mixing produced with two ultrashort phase-locked !-3! laser pulses propagating coherently in a two-level system in the infrared spectral region is shown to depend on the pulses relative phase. The Maxwell-Bloch equations are solved beyond the rotating-wave approximation to account for field frequencies which are largely detuned from the atomic resonance. The relative phase dominating the efficiency of the coupling to the 5! anti-Stokes Raman component is determined by sign of the total ac Stark shift induced in the system, in such a way that the phase influence disappears precisely where the ac Stark effect due to both pulses is compensated. This fundamental quantum interference effect can be the basis for nonlinear ultrafast optical spectroscopy techniques. Phenomena arising from the coherent control of nonlinear interactions are of importance in fields as diverse as optoelectronics and materials research, in high harmonic generation, in photoionization or molecular dissociation, and in biological applications such as spectroscopy and imaging, among others [1, 2, 3, 4, 5, 6, 7]. Optical quantum coherent control is based on the fact that the phases of interfering transition amplitudes in light-matter interactions can be controlled through the optical phase of coherent light sources that drive the interaction, in such a way that the transition rates to final states and the dynamics at various stages of the process can be modified [1]. In a recent paper [8], a theoretical investigation on the quantum coherent control of the optical transient four-wave mixing of two intense phase-locked femtosecond laser pulses of central angular frequencies ω and 3ω propagating in a two-level atom (TLA) was reported. It was shown how the nonlinear (χ (3) ) coupling to the antiStokes Raman field at frequency 5ω depends critically on the initial relative phase φ of the propagating pulses. In Ref. [8], the study was centered to intense pulses in the visible and ultraviolet spectral regions, with frequencies at resonance or lower than the atomic transition. The phenomena observed in [8], however, can be scaled to various laser and material parameters. In the infrared spectral range, for instance, experiments on ultrafast molecular dynamics are frequently performed by help of twocolor pump probe nonlinear spectroscopy techniques. In this type of studies, due to the high intensities inherent to ultrashort (subpicosecond) pulses, nonlinear effects such as stimulated Raman processes may become important and are often utilized as a complementary tool to gain information [9]. In this Letter, we address the conditions for quantum coherent control of transient four-wave mixing interactions with two phase-locked ω-3ω femtosecond laser pulses in the mid-IR spectral region. Our purpose is to reveal the basic physics for a strict two-level medium and to this end the Maxwell-Bloch TLA will be considered. We will study pulses with a duration of 300 fs (with a w 5w